The invention relates to a device for automatically releasing the mobile contacts of a cut-off apparatus adapted for breaking a circuit under load such as a contractor adapted for limiting short circuit currents and more to such devices which make use of the electro-dynamic repulsion forces which appear between conductors when a current is flowing therethrough. In such a device, a mobile contact holder has a longitudinal coupling piece which is integral with a contact bridge subjected to these repulsion forces and which is connected to an operating member through a threshold coupling. The threshold coupling comprises convex-shaped members which cooperate transversely with respect to the longitudinal axis and resiliently with notches for providing a predetermined longitudinal holding force between the bridge and the operating member.